Limitations & Problems

Limitations & Problems: Although LWA1 is science-ready, we are currently operating on a “shared risk” basis and there are known issues that prospective users should know about:

Beam pointing error is now automatically corrected to within a few tenths of a degree by the monitoring and control system. See LWA Memo 194 for a description and history of this issue.

Beam sensitivity, which was degraded by about 50% due to bad cable delay recalibration, affected observations from August 2012 through February 2013. A recalibration has restored the original sensitivity, and may also have reduced the sidelobes somewhat.

Beam gain uncertainty on the order of a few percent. For example, the beam output power when tracking a strong source will appear to vary on the order of a few percent with a period of about 15 minutes. Thus, the efficacy of off-pointing for flux calibration is limited.

Sidelobes can be very high; typically above -20 dB but perhaps as high as -10 dB.

Timetag offsets and glitching: Time information embedded in the data stream may in some cases contain an offset on the order of seconds, and in certain cases may “glitch” forward or backward. The data itself is not affected. The problem is associated with frequent retuning (changing center frequencies); since most observations dwell on a center frequency this is usually not a problem. LSL has diagnostic tools that can detect this issue; consult Jayce Dowell for details.

Concurrence of beam and transient buffer modes. Running beams (formally “DRX-mode sessions”) and “all dipoles” modes (TBN or TBW) concurrently can be risky. If you want or need to do this, consult with a staff member.

New issues are being identified all the time, and old ones are being solved. Users are encouraged to consult with a staff member to make sure they have the latest information.